US investigators probing Mercedes maker Daimler have found that its cars were equipped with software that may have helped them to pass diesel emissions tests, a German newspaper reported Sunday, citing confidential documents.

There has been growing scrutiny of diesel vehicles since Volkswagen admitted in 2015 installing secret software on 580,000 US vehicles that allowed them to emit up to 40 times legally allowable emissions while meeting standards when tested by regulators.

Daimler, which faces ongoing investigations by US and German authorities into excess diesel emissions, has said investigations could lead to significant penalties and recalls.

The Bild am Sonntag newspaper said the documents showed US investigators had found several software functions that helped Daimler cars pass emissions tests, including one which switched off emissions cleaning after roughly 16 miles of driving.

Another function under scrutiny allowed the emissions cleaning system to recognize whether the car was being tested based on speed or acceleration patterns.

The German newspaper also cited emails from Daimler engineers questioning whether these software functions were legal.

A Daimler spokesman declined to comment on the content of the documents, saying the automaker was fully cooperating with US authorities and had agreed upon strict confidentiality with the Department of Justice.

“The authorities know the documents and no complaint has been filed,” the spokesman said. “The documents available to Bild have obviously selectively been released in order to harm Daimler and its 290,000 employees.”